Accidentally pressed F12, and lost rendered image. Is there way to recover it?
Unless you had saved your image to a file, there is no way to get your rendered image back.

Things you can do to prevent this from happening in the future:


Save the rendered image to a file:

In the UV/Image editor's menu use Image->Save As Image (or F3)


Switch render slots.



There are 8 independent render slots. If you switch to a different one and render again, the images on other render slots will remain unaffected. Note that you'll be increasing memory usage with every render slot, and all of the rendered images will be lost when the file is closed (unless you save them to an external file).